As an energy source in place of fossil fuel, solar cells capable of converting sun light to electric power have drawn attention. Presently, some of solar cells using crystalline silicon substrates and thin film silicon solar cells have been used practically. However, the former has a problem of a high production cost of the silicon substrates and the latter has a problem that the production cost is increased since various kinds of gases for semiconductor production and complicated production facilities are required. Therefore, in both solar cells, it has been tried to lower the cost per electric power output by increasing the efficiency of photoelectric conversion; however, the above-mentioned problems still remain while being unsolved.
As a new type solar cell, there has been proposed a wet type solar cell based on photo-induced electron transfer of a metal complex (see Japanese Patent No. 2664194; Patent Document 1).
This wet type solar cell comprises: two glass substrates each of which has an electrode on a surface thereof; and a photoelectric conversion layer which contains a photoelectric conversion material having an absorption spectrum in a visible light region by adsorbing a photo-sensitive dye and an electrolytic material and which is sandwiched between the electrodes of two glass substrates. Specifically, as shown in FIG. 9, the dye-sensitized solar cell is produced by injecting an electrolytic solution between two glass substrates. In the drawing, a reference numeral 100 denotes a first support (glass substrate); a reference numeral 101 denotes a second support (glass substrate); a reference numeral 102 denotes a conductive layer; a reference numeral 103 denotes a sealing material; a reference numeral 104 denotes a photoelectric conversion layer; a reference numeral 105 denotes a catalyst layer; a reference numeral 106 denotes a counter conductive layer; and a reference numeral 107 denotes a carrier transporting layer (electrolytic solution).
When the wet type solar cell is irradiated with light, electrons are generated in the photoelectric conversion layer, the generated electrons transfer to the electrodes through an external electric circuit, and the transferred electrons are conveyed to the electrodes opposed owing to the ion in the electrolytic material and turn back to the photoelectric conversion layer. Owing to the series of the flow of the electrons, electric energy is outputted.
However, since a basic structure of the dye-sensitized solar cell described in Patent Document 1 is a structure that the electrolytic solution is injected between the opposed transparent conductive film-bearing glass substrates, it is possible to produce a trial solar cell with a small surface area, but it is difficult to practically produce a solar cell with a large surface area such as 1 m square. That is, if one solar cell is enlarged in the surface area, the generated current is increased proportional to the area. However, since a voltage decrease in the plane direction of the transparent conductive film to be used for the electrode parts is increased, the inner resistance in series of the solar cell is increased. As a result, FF (fill-factor) and a short circuit current at the time of the photoelectric conversion are lowered, resulting in a problem of decrease of the photoelectric conversion efficiency.
Further, since the dye-sensitized solar cell module is produced by forming elements between the opposed transparent conductive film-bearing glass substrates, the module has problems that the production cost is increased and the weight is increased.
In order to solve the problems on the inner resistance in series, there has been proposed a dye-sensitized solar cell module having a plurality of dye-sensitized solar cells connected in series (see Japanese Unexamined Patent Publication No. 2002-540559: Patent Document 2).
In this dye-sensitized solar cell module as shown in FIG. 10, a glass substrate 110 bearing a transparent conductive film (electrode) 112 formed in a comb-like shape by patterning and a glass substrate 111 bearing a transparent conductive film (electrode) 116 and a catalyst layer 115 formed successively in a comb-like shape by patterning are stuck to each other in a manner that an insulating layer 113 is interposed between the glass substrates so as to form respective dye-sensitized solar cells and also a conductive path (contact layer) 118 for electrically connecting a catalyst layer 115 and the transparent conductive films 112 and 116 is arranged so as to connect neighboring dye-sensitized solar cells in series and furthermore a photoelectric conversion layer 114 and an electrolytic solution 117 are sandwiched between the glass substrates.
Further, a dye-sensitized solar cell module having W-type series connection proposed by P. M. Sommeling et al., is described in “Development Technology of Dye-Sensitized Solar Cells”, edited by HAYASE Shuji and FUJISHIMA Akira, Gijutsu Kyoiku, p. 205-217, June 2003 (Non-Patent Document 1).
In this dye-sensitized solar cell module as shown in FIG. 11, a porous semiconductor layer which is a photoelectric conversion layer 214 and platinum which is a catalyst layer 215 are alternately formed on two glass substrates 210 and 211 bearing transparent conductive films (electrodes) 212 and 216 formed in a comb-like shape by patterning and are stuck to each other in a state that the porous semiconductor layers and the platinum on the respective glass substrates are arranged face to face and in a manner that an insulating layer 213 of a resin, or the like, is interposed between the substrates so as to form each dye-sensitized solar cell and an electrolytic solution 217 is sandwiched between the substrates.
However, the dye-sensitized solar cell modules described in Patent Document 2 and Non-Patent Document 1 have a configuration that a basic structure of each dye-sensitized solar cell is formed by injecting the electrolytic solution between the opposed transparent conductive film-bearing glass substrates, and therefore the problems of the production cost and weight still remain while being unsolved.
Accordingly, in order to solve the problems of the production cost and weight, there has been proposed a dye-sensitized solar cell module having one transparent conductive film-bearing glass substrate and a plurality of dye-sensitized solar cells (sometimes referred to as “photovoltaic cells”) connected in series and arranged on the glass substrate (e.g., see International Publication WO 97/16838: Patent Document 3).
In the dye-sensitized solar cell module as shown in FIG. 12, each dye-sensitized solar cell has a structure formed by successively layering a porous semiconductor layer (porous titanium oxide layer) 314 which is a photoelectric conversion layer, a porous insulating layer (intermediate porous insulating layer) 318, and a counter electrode 315 on a transparent substrate (glass substrate) 310 bearing a transparent conductive film (electrode) 312 formed in a comb-like shape by patterning and the dye-sensitized solar cells are arranged in a manner that the transparent conductive film 312 of one dye-sensitized solar cell and the counter electrode 315 of a neighboring dye-sensitized solar cell are brought into contact with each other, and thus both solar cells are connected in series. In the drawing, a reference numeral 311 denotes a top cover for tightly sealing an insulating liquid and a reference numeral 313 denotes an insulating layer.
Further, Japanese Unexamined Patent Publication No. 2002-367686 (Patent Document 4) discloses a dye-sensitized solar cell module having an integrated structure including a transparent conductive film, a porous semiconductor layer, a porous insulating layer, and a catalyst layer formed on a transparent substrate. This technique determines a particle diameter of component particles of each of the porous semiconductor layer, the porous insulating layer, and the catalyst layer, and thus can prevent particles of formed layers from being mixed in the porous layers each of which is an under layer when each layer is formed.
However, the dye-sensitized solar cell modules described in Patent Documents 3 and 4 are required to successively layer the porous semiconductor layer, the porous insulating layer, and the catalyst layer on one transparent conductive film-bearing glass substrate and fire the respective layer after formation of the respective layers. Therefore, the processing steps are increased and the transportation resistance of a carrier transporting material is increased due to the formation of the porous insulating layer, resulting in a problem of deterioration of the performance of the solar cell.
In general, when a catalyst layer and a porous semiconductor layer of a photoelectric conversion layer are brought into contact with each other, leakage, that is, electron injection from the photoelectric conversion layer to the catalyst layer and a counter conductive layer, occurs in the contact part. In order to prevent the leakage, it is preferable to form a Schottky barrier between the catalyst layer and the porous semiconductor layer. Accordingly, at least the catalyst layer among the counter constituent elements is preferable to have a lower work function than the conduction band energy level of the porous semiconductor layer and thus activate a redox reaction of the carrier transporting material.
As a material for forming such a catalyst layer, for example, platinum (work function: 6.35 eV) is preferable in the case of using titanium oxide (electron affinity=conduction band energy level: 4.1 eV) for the porous semiconductor layer. However, if the porous semiconductor layer and the catalyst layer are formed using fine particles, the physical values (energy levels and Schottky barrier between two type materials) of the respective materials cannot often be applied as they are. For example, in a decomposition step such as deodorization by utilizing a photocatalytic function of titanium oxide, there is a technique of increasing the photocatalytic function of titanium oxide by supporting platinum particles in a size of several nanometers on titanium oxide fine particles in a size of several ten nanometers and it is known well that electrons are shifted from titanium oxide to platinum.
Therefore, in the dye-sensitized solar cell module and dye-sensitized solar cell described in Patent Document 3 and 4, the porous insulating layer using a material with a high conduction band energy level such as zirconium oxide is formed on the porous semiconductor layer.
On the other hand, in order to make the best use of incident light to the dye-sensitized solar cell, there is a technique of forming a porous semiconductor layer in a layered state using fine particles with various particle diameters and it is confirmed that this technique can improve the performance of the solar cell (see, Japanese Unexamined Patent Publication No. 2001-93591, Patent Document 5).
In this porous semiconductor layer as shown in FIG. 13, a conductive layer 22, a porous semiconductor layer 23 adsorbing a dye, and a catalyst layer (not illustrated) are successively layered on a support 21 in an incident light (light receiving face) side. Further, in the porous semiconductor layer 23, semiconductor particles 24 with a smaller particle diameter and semiconductor particles 25 with a larger particle diameter are layered in this order, that is, in an order of from a layer with lower light scattering property to a layer with higher light scattering property, from the light receiving face side.
With respect to such a porous semiconductor layer, the light absorption probabilities in each layer of the porous semiconductor layers 23 differ. That is, the incident light from a support 21 side is successively absorbed by a dye adsorbed in the porous semiconductor layer 23 formed on the conductive layer 22 and proceeds in a catalyst layer (not illustrated) direction. Since such light absorption step is carried out in the inside of the solar cell, the dye adsorbed on a portion of the porous semiconductor layer 23 close to the support 21 most absorbs light, and as the light proceeds in the catalyst layer (not illustrated) direction, the amount of incident light is decreased more and the photoelectric conversion per unit time is lowered more.    Patent Document 1: Japanese Patent No. 2664194    Patent Document 2: Japanese Unexamined Patent Publication No. 2002-540559    Patent Document 3: International Publication WO 97/16838    Patent Document 4: Japanese Unexamined Patent Publication No. 2002-367686    Patent Document 5: Japanese Unexamined Patent Publication No. 2001-93591    Non-Patent Document 1: “Development Technology of Dye-Sensitized Solar Cells”, edited by HAYASE Shuji and FUJISHIMA Akira, Gijutsu Kyoiku, p. 205-217, June 2003